It is difficult to obtain precise indications of flow rate through a variable area metering valve merely by measurement of the actual geometric opening of the valve not only because different types of valves have different flow characteristic curves relating percentage flow through the valve against percentage opening of the valve, as illustrated in Millar U.S. Pat. No. 4,581,707 granted Apr. 8, 1986, but changes in pressure and temperature alter the flow performance. In some installations, the prior art has employed a display to indicate whether the valve was closed or open together with a flow meter to determine flow rate. In Molusis U.S. Pat. No. 4,149,254 issued Apr. 10, 1979, a system is disclosed for periodically calculating gas being utilized while it is flowing through an orifice and for displaying operating and test parameters.